'Rudder' is Old English for 'steering oar' — from PIE *reh- (to row). The helm's oldest ancestor.
A flat piece of wood, metal, or fiberglass hinged vertically to the stern of a boat or ship, turned to steer the vessel; by extension, any device that provides directional control.
From Old English 'rōþor' (a paddle, a rudder, an oar used for steering), from Proto-Germanic *rōþrą (a rudder, a steering oar), from PIE *h₁reh₁- (to row). The word is cognate with Old Norse 'róðr' (rowing), Old High German 'ruodar' (an oar), and Dutch 'roer' (a rudder, a helm). The original sense was a steering oar — a large oar mounted at the stern and used to control direction — which evolved into the hinged
The sternpost rudder — hinged to the back of the ship rather than operated as a side-mounted oar — was one of the most transformative inventions in maritime history. It appeared in China by the first century CE and in Europe by the twelfth century. Without it, large sailing ships capable of oceanic voyages would have been uncontrollable.